The invention relates to a contact-free determination of distances and thicknesses wherein the distance of a point on a surface of an object from a zero plane is measured by use of a sharply focused light beam which is periodically deflected by means of a beam deflector over a measuring area in which the object is arranged. A measuring photo detector is provided which is able to receive light only from a line of sight and with which it is possible to determine a measuring time at which the point located on the line of sight of the object's surface is hit by the light beam. The distance is determined from the time difference between the measuring time and the time at which the light beam is directed towards the point on the zero plane located on the light of sight.
Processes of this type are described, for example, in the German Offenlegungsschrift No. 2,157,813 and the German Offenlegungsschrift No. 2,546,714 and in the German Pat. No. 2,501,015 corresponding to U.S. Pat. No. 4,068,955.
A common feature of these processes and devices is that the thickness is determined by measuring the distances from the upper and lower sides of the object to be measured to a virtual zero plane. The number of distances between the upper and lower sides of the object to be measured to the virtual zero plane which must be established in order to determine the thickness is dependent upon the position of the object to be measured relative to the zero plane. For plane-parallel objects which lie or move in parallel with the zero plane, it is sufficient to measure a distance from the upper and lower sides of the object to be measured to the zero plane. The thickness of the object is then determined by subtracting the two distances. Determining the thickness of objects to be measured tilted relative to the zero plane is more complicated. In this case there is no linear relationship between the thickness and the measured distances.